Heretofore, as this kind of materials for gasket, a material, in which gum foam layers 3 are formed on the two surfaces of a metal plate 1 through primer layers 2 and a non-adhesive layer 4 is formed on the surface of each of the gum foam layers, as indicated in the figure is widely utilized.
Since the material for gasket constructed as described above has a great compression resiliency owing to the gum foam layers formed by independent or continuous bubbles, no embossing operation (formation of beads) is required for gasket consisting of gum layers formed on the surfaces of a metal plate, but it is possible to obtain gasket products of high quality only by punching it in a predetermined shape.
However the material for gasket constructed as described above has problems as described below.
(A) The material for gasket, in which the gum foam layers are formed on the metal plate, has drawbacks that, since the gum layers serving as sealing layers are foamed, it is short in properties of matter such as tensile strength, stress relaxation, etc. and stress relaxation required for a sealing member is great, and that it can give rise to blowing out of the foam layers at a high pressure sealing, which takes place due to the fact that the strength of the gum foam layers is low, and to wear and shear deformation destruction of the gum form layers due to thermal contraction movement of an opposite flange member.
(B) In a gum form layer formed by using a prior art thermally decomposing foaming agent, the form cell diameter is great and further since it is formed by the normal pressure foaming method, independent bubbles and continuous bubbles exist mixedly. For this reason it has a drawback that fluid penetration into the gum foam layers through continuous bubble parts and stress relaxation of the gum form layer are great, which gives rise to decrease in the sealing property at a low pressure at the surface thereof.